The present invention relates to a multi-carrier transmitting apparatus and method for a radio network, wherein a plurality of carrier signals are combined in the digital domain.
In recent years, increased research and development has taken place in the field of multi-carrier wideband applications for FDMA, TDMA or CDMA systems. Such multi-carrier applications provide potential cost and size benefits.
FIG. 3 shows a known multi-carrier transmitter, wherein signal combining of multiple carriers is performed in the digital domain before a subsequent D/A-conversion.
In FIG. 3, reference numerals 11 to 14 indicate base band processing means for processing digital base band signals to be transmitted on different carrier frequencies. Therein, different input signals are placed on different carrier frequencies by a bank of parallel DDSs (Direct Digital Synthesizers) 21 to 24 as a means for digitally generating carrier signals which can be modulated easily. Alternatively, a Fast Fourier Transformation (FFT) could be performed on the input signals. If the DDSs 21 to 24 are used, modulation can be combined in the digital frequency synthesis. In case of FFT, the modulation is performed prior to frequency transformation.
Subsequently, parallel words output from the DDSs 21 to 24 and representing the modulated digital signals are combined by a digital addition and supplied to a D/A converting means 3. In case of the FFT process a single bit stream is generated, such that an addition is not required.
The D/A converting means 3 performs a D/A-conversion of the digital combined multi-carrier signal so as to generate an analog multi-carrier signal which is supplied to a transmitting means 4. In the transmitting means 4, the analog multi-carrier signal received from the D/A converting means 3 is frequency-converted to the final transmission frequency in order to be transmitted via corresponding radio channels.
However, the output amplitude response of the D/A converting means 3 exhibits a sinc(x) characteristic as shown in FIG. 4. Such a sinc(x) characteristic is defined by the following equation:
A(f0)=sin(xcfx80f0/fc)/(xcfx80f0/fc),
wherein f0 denotes an output frequency and fc a clock frequency of the D/A converting means 3.
In the above case of the multi-carrier transmitter, the multiple carrier signals are combined in the digital domain before the D/A-conversion and are located at different frequencies. Thus, the output amplitudes of different carriers will be weighted by the above output amplitude response or transfer function of the D/A converting means 3.
FIG. 5a shows a frequency spectrum of the digital multi-carrier signal before the D/A-conversion. In the shown case, the combined four carrier signals have the same absolute value |A| of the amplitude.
If a digital multi-carrier signal having such a frequency spectrum is input into the D/A converting means 3, a D/A converted output signal is obtained having a frequency spectrum as shown in FIG. 5b. According to FIG. 5b, the absolute values |A| of the amplitudes of the individual carrier signals have been weighted by the transfer function of the D/A converting means 3, wherein the absolute value |A|decreases with increased carrier frequencies.
According to a known solution to the weighting problem of the D/A converting means 3, an analog filter having a 1/sinc(x) frequency response is provided after the D/A converting means 3. Such a 1/sinc(x) frequency response is shown in FIG. 6.
Thus, the analog filter serves to compensate for the weighting performed by the D/A converting means 3.
However, such an analog filter exhibits non-idealities and thus degrades the signal as well as adds to the amount of analog hardware.
According to another known solution, a digital filter with a 1/sinc(x) frequency response is inserted after combining the individual carriers, but before the D/A converting means 3. FIG. 7 shows the frequency response of such a digital filter.
Digital filtering or pre-emphasis provides higher performance than the analog filter, but at the cost of increased signal processing. Furthermore, the processing has to take place at a high date rate, which is typically equal to the clock rate of the D/A conversion.
It is an object of the present invention to provide a multi-carrier transmitting apparatus and method, by means of which the described amplitude distortion of the multiple carrier signal can be compensated in a simple manner.
This object is achieved by a multi-carrier transmitting apparatus comprising: scaling means for pre-scaling individual ones of a plurality of carrier signals; combining means for combining said plurality of carrier signals in the digital domain; and D/A converting means for converting the combined digital carrier signals into an analog signal to be transmitted, wherein a scaling factor of said scaling means is selected so as to compensate for a frequency characteristic of said D/A converting means.
Furthermore, this object is achieved by a multi-carrier transmission method comprising the steps of: pre-scaling individual ones of a plurality of carrier signals; combining said plurality of carrier signals in the digital domain; and D/A converting the combined carrier signals into an analog signal to be transmitted, wherein a scaling factor used in the pre-scaling step is selected so as to compensate for a frequency characteristic of the D/A-conversion step.
Accordingly, the inconvenient digital or analog filter can be replaced by a simple scaling of each individual carrier. This takes place before combining the carriers. Thereby, a signal processing intensive digital filter or a non-ideal analog filter is not required.
Moreover, the pre-scaling can be done at a low data rate, which typically equals the symbol rate, before data interpolation. For example, in a GSM system with a multicarrier signal having a bandwidth of 10 MHz, the clock rate must be larger than 20 MHz, preferably larger than 30 MHz, whereas the symbol rate is only 271 kHz.
Preferably, the scaling is performed by multiplying the digital carrier data by the scaling factor. Therein, a plurality of frequency-dependent scaling factors can be stored in a storing means of a control means used for controlling the transmitting apparatus. The control means may select and supply the scaling factor to the scaling means. Preferably, the storing means may comprise a look-up table including the plurality of frequency-dependent scaling factors.
In case a digital power control is provided, the power control words may also be used for performing the pre-scaling. In such a case, the control means may comprise a power control means for supplying power control words to be used for controlling the power of the plurality of carrier signals, and a scaling control means for supplying frequency-dependent scaling control words to the scaling means, wherein the control means is arranged to multiply each of the plurality of power control words with the corresponding one of the plurality of scaling control words and to supply the results to the scaling means.
Thereby, the hardware requirements are minimized, since the power control means can be extended so as to perform pre-scaling as well.
Further preferred developments of the present invention are defined in the dependent claims.